Conventional single vision lenses do not adequately correct for optical aberrations apparent to the wearer when viewing objects, through the lens, at oblique angles. This results from the lens surfaces not being the ideal shape to fully correct for the wearer's specific needs of power, cylinder correction and fitting requirements for off-axis vision.
Attempts have been made to correct such errors in the prior art with limited success, and have most often resulted in compromises that either do not fully correct the entire lens surface or that inadequately address the optical aberrations and errors.
Typically, prior art attempts introduced corrections by aspherising one or both of the lens surfaces for spherical or toric corrections. For example, typical prior art methods have relied on aspherising the two principle toric meridians of the lens back surface, improving the optical performance of these meridians independent of each other. This method has proven to be deficient in that the regions between the two principle meridians are not sufficiently optically optimised in the same way as the vision along the principle meridians.
Other methods have typically included minimising astigmatism on a grid of points on the lens surface using optical ray tracing techniques, following which a twice continuously differentiable spline surface (satisfying the data at each point) is fitted. However, it is only possible to satisfy these conditions for a limited range of simple surfaces that cover only a fraction of all surfaces that are of interest to the ophthalmic industry. In other cases, such an approach normally requires the fitting of a smoothing spline surface in an attempt to approximate the calculated data at each point, without satisfying it exactly. As a result, the optics of such a surface often significantly deviates from the optimal case.
It is an object of the present invention to provide an improved single vision ophthalmic lens that overcomes the difficulties identified above with prior art single vision ophthalmic lenses.